1. Field of the Invention
The present invention relates to a magnet wire having a high heat resistance, i.e., resistance to at least 200.degree. C., and to a method of removing an insulating covering from the magnet wire having a high heat resistance.
2. Description of the Related Art
In recent years, electronic appliances tend to become smaller and smaller in size and lighter and lighter in weight, with the result that a high heat resistance is required in a magnet wire used for forming, for example, a coil. Resin materials adapted for use in the manufacture of a magnet wire having a high heat resistance include, for example, polyimide, polyamideimide, polyparabanic acid, polyhydantoin, polyesteramideimide, and polyesterimide.
Where the insulating covering of the magnet wire of this type is removed for the terminal processing of the magnet wire, it was customary to employ a mechanical method using, for example, a wire stripper or a sand paper. It is also known to the art to burn away the insulating covering. In the conventional technique, however, a serious damage is done to that region of the conductor of the magnet wire from which the insulating covering is removed, resulting in failure to ensure a sufficient reliability when the wires are connected to each other or the wire is connected to the terminals of parts.
A measure for overcoming the difficulty inherent in the prior art is proposed in, for example, Published Unexamined Japanese Patent Application No. 49-97288. The magnet wire proposed in this prior art comprises an insulating covering of a laminate structure consisting of an outer layer formed of polyimide or polyamideimide and a inner layer formed of a polyesterimide which does not contain an aromatic trihydric alcohol having relatively low heat resistance. It is taught that a soldering treatment can be applied directly to the magnet wire proposed in this prior art without removing the insulating covering. Because of the high solderability, the terminal processing can be markedly facilitated. However, in the insulating covering included in the magnet wire proposed in this prior art, the outer layer having a relatively high heat resistance is formed thin, with the inner layer having a relatively low heat resistance being formed thick. Because of the particular construction, the magnet wire is not satisfactory in heat resistance. To be more specific, the magnet wire proposed in JP '288 noted above fails to exhibit a heat resistance of 200.degree. C. or more specified in IEC (International Electrotechnical Commission) Pub. 172. Also, when an excess current flows through the magnet wire, the resin material in the inner layer of the insulating covering is decomposed and foamed, with the result that the outer layer tends to be broken so as to bring about a short-circuit problem.
On the other hand, it is impossible to apply a soldering treatment directly to a magnet wire without removing the insulating covering, when it comes to a magnet wire comprising an insulating covering which exhibits a heat resistance of 200.degree. C. or more specified in IEC Pub. 172. Naturally, the terminal processing of the magnet wire is made troublesome.
As described above, it is impossible to obtain presently a magnet wire which can be subjected to a terminal processing without difficulty and which comprises an insulating covering capable of exhibiting a heat resistance of at least 200.degree. C. specified in IEC Pub. 172. Such being the situation, a laser-removing method, in which the insulating covering is irradiated with a laser beam for removing the insulating covering, attracts attentions as a method which readily permits a terminal processing of a magnet wire without doing damage to the conductor of the magnet wire. The laser-removing method is described in various publications including, for example, Published Unexamined Japanese Patent Application No. 59-25509, Published Unexamined Japanese Patent Application No. 1-295609, Published Unexamined Japanese Patent Application No. 2-17813, Published Unexamined Japanese Patent Application No. 2-155412, Published Unexamined Japanese Patent Application No. 2-197206, Published Unexamined Japanese Patent Application No. 3-212109, Published Unexamined Japanese Patent Application No. 4-17989, Published Unexamined Japanese Patent Application No. 4-105509, and Published Unexamined Japanese Patent Application No. 4-100652.
The present inventors have paid attentions as an effective method to the terminal covering removing method using an excimer laser. It has been found that, where the excimer laser method is applied to a magnet wire of the conventional construction, it is certainly possible to remove the insulating covering in the first irradiation. However, it has also been found that the insulating covering is decomposed to generate carbon, when the insulating covering is exposed to the excimer laser beam, with the result that the generated carbon is attached to the periphery of the laser-irradiated region so as to inhibit the removal of the insulating covering in the second laser beam irradiation et seq.
It has been found through further researches that a CO.sub.2 laser, particularly a pulse oscillation type CO.sub.2 laser, is most adapted for the removal of the insulating covering. However, the treatment with the pulse oscillation type CO.sub.2 laser has been found to take at least as much as 15 seconds for removing the insulating covering from a magnet wire exhibiting a heat resistance of at least 200.degree. C. specified in IEC Pub. 172, e.g., a magnet wire comprising a polyimide insulating covering. It follows that the treatment with a CO.sub.2 laser beam is unsuitable for the practical application in the industry.
Incidentally, it is described in Published Unexamined Japanese Patent Application No. 3-212109 that an insulating covering of a magnet wire having a high heat resistance is removed by the treatment with a pulse oscillation type CO.sub.2 laser beam so as to permit a soldering treatment. In this method, however, it is necessary to apply pre-treatments such as heating or immersing in water to the magnet wire. It is also necessary to increase very much the irradiation density of the laser beam. In the Example described in this prior art, the laser beam irradiation density is as high as 72 J/cm.sup.2 or more. Further the magnet wire is disposed at a position closer to a converging lens than the focus of the laser beam. What should also be noted is that the method disclosed in this prior art is dangerous. Specifically, since the irradiation density of the laser beam is very high in this method, spark is likely to take place in the focus of the laser beam. Further, the insulating covering is removed in only a very small region. Also, since a pre-treatment is required, the operability is very low.